1. Field of Invention
The present invention relates to light module. More particularly, the present invention relates to a backlight module, suitable for use in liquid crystal display (LCD).
2. Description of Related Art
Recently, the technology of the LCD panel has been well developed. Now, the LCD panel has been quite popular in the daily life for information communication, such as TV, display screen, computer terminal . . . and so on. Since the liquid crystal does not generate the light by itself, the LCD needs a light module as a light source to display the color image; the LCD is usually implemented with a light module. The backlight module is one of the light modules, used in LCD to provide a planar light source at the back of the LCD panel. In other words, the LCD panel has the displaying part and a light module. The displaying part can be the assembly structure in state-of-the-art other than the light module. A top view of a conventional backlight module is schematically shown in FIG. 1. In FIG. 1, the conventional backlight module includes a plurality of lamps 100, which are located above a grounded back plate 102. The lamps 100 are arranged one by one in parallel to the grounded back plate 102, so as to produce a uniform planar light source. The lamps 100 usually are cold cathode fluorescent light, and the back player usually is an A1 plate being grounded. The backlight module also include other elements, such as reflector, holding frame . . . and so on, which should be known by the ordinary skilled artisans and are not further described here.
The lamp 100 are typically applied a bias between a high voltage and a low voltage, so as to produce current and trig the light. However, since the lamp 100 is located above the grounded back plate 102, a leakage current is not avoidable. FIG. 2 is a drawing, schematically illustrating the mechanism of leak current for the conventional backlight module. In FIG. 2, the lamp 100 is separated from the back plate 102 by a distance D, such as 1 mm-2 mm. In electric property, there are many equivalent parasitic capacitors existing between the lamp 100 and the back plate 102. The lamp 100 has high voltage end Hi at a current level of IL-1 and a low voltage end Lo at a current level of IL-2. The length of the lamp 100 is L, then the current at the beginning end at the level IL-1, and is gradually leaks to the grounded back plate 102 along the length. The dropping amount is due the leak current. The actual current at the low end is at the level of IL-2. If the leak current is small, the current should remain at the level IL-2′. The leak current reduces the luminance of the lamp 100.
FIG. 3 is a drawing, schematically illustrating the phenomenon of leak current in more detail. In FIG. 3, as one can see, when the light is turned on, the current in a short time period is triggered by the applied bias. Then, the current flows toward to the low voltage end in the time period. However, due to the leak current, the current gradually leaks to the grounded back plate in the time period.
According to the electric property, if the distance D is larger, then the capacitance is smaller. In other words, the distance D is infinity, then the lamp is isolated, and there is no leak current. FIG. 4 is a drawing, schematically illustrating two parameters, which are related to the quantity of leak current. One parameter is the distance D as shown at the left curve and the other parameter is applied bias V as shown at the right curve. Since the bias V consumes the power, the designer would like to keep the operation bias as low as possible. Then, the parameter D may be considered for improving the luminance of the lamp at the same operation bias. However, a large distance D certainly causes a large thickness of the light module, causing the increase thickness of the LCD. The distance D cannot be freely increased to solve the leak current. The technology is still under developing by the manufacturers, so as to increase the effective luminance without increasing the operation voltage.
In further consideration, U.S. Pat. No. 6,839,100 has disclosed an LCD device with a longitudinal lamp at side. In order to hold the lamp, two lamp holders enclose the both electrode ends of the lamp. Then, a lower frame usually in contact with the lamp holder, so that the heat at the both electrode ends is dispersed to the lower frame through the lamp holder, causing an undesired temperature drop at the both electrode end for the fluorescent lamp. In order to solve this thermal issue, the lower frame is designed with two openings near the both electrode ends. As a result, the lamp holder is not contact with the lower frame, and the opening of the lower frame may solve the thermal effect. However, the electric property, such as the leakage current as discussed above is not considered. U.S. Pat. No. 6,839,100 does not solve the electric issues discussed above and the structure of the present invention mentioned below.